1. Field of the Invention
The present invention relates to a wiring body connection structure electrically connecting a flexible extendable and contractible wiring body that utilizes an elastomer and another wiring body connectable to a connector on a circuit board.
2. Description of Related Art
Flexible sensors, actuators and the like have been developed utilizing elastomer. For flexible sensors and actuators, it is required that an electrode and a wiring are able to follow deformation of a base material, a dielectric film and the like that are made from an elastomer. For example, a capacitive sensor can be formed by arranging a pair of electrodes on front and rear surfaces of a dielectric film made from an elastomer. In this case, when a load is applied to the sensor, the dielectric film deforms. In this case, it is desirable that the electrodes are extendable and contractible in accordance with the deformation of the dielectric film so as not to impede the deformation of the dielectric film. Similarly, it is desirable that wirings connected to the electrodes are extendable and contractible following the deformation of the dielectric film and the electrodes. Therefore, attempts have been made to form electrodes and wirings from a conductive material that combines powders of a conductive carbon or a metal with an elastomer (for example, see Related Arts 1 and 2).
In the above described flexible sensor and the like, one end of a wiring is connected to an electrode and the other end is connected to an electrical circuit of a control device or the like. However, a method allowing a stable connection between an extending and contracting flexible wiring and an electrical circuit has not yet been established. On the other hand, as a method of making an electrical connection between terminals of an existing circuit board such as a flexible printed circuit board (FPC), an anisotropic conductive adhesive and the like have been used (for example, see Related Arts 3 and 4).
[Related Art 1] Japanese Patent Laid-Open Publication No. 2010-43880
[Related Art 2] Japanese Patent Laid-Open Publication No. 2007-173226
[Related Art 3] Japanese Patent Laid-Open Publication No. HEI 5-218634
[Related Art 4] Japanese Patent Laid-Open Publication No. HEI 5-25446
When a flexible wiring body is made by forming a wiring (which is made of the above described conductive material) on a front surface of a base material (which is made of an elastomer) and the flexible wiring body is connected to an electrical circuit, a possible method is to directly connect the wiring body and an existing connector provided on a circuit board. According to the existing connector, an electrode of the connector bites into a wiring body to electrically connect the wiring body and the electrical circuit. However, as described above, the wiring extends or contracts following the deformation of the connecting electrode and the dielectric film. When extension and contraction are repeated, settling occurs in the wiring due to compression set of the elastomer. In this case, in a connection by mechanical biting between the wiring body and the connector, the connection portion cannot follow the settling of the wiring. As a result, there is a risk that a contact failure may occur between the wiring body and the connector. The base material of the wiring body is made from an elastomer. The wiring also uses an elastomer as a parent material. Therefore, the wiring body has a relatively small mechanical strength. Therefore, due to biting by the connector, there is a risk that cracking may occur in the wiring and the like. As just described, when a flexible wiring body that utilizes an elastomer is connected to an existing connector, there is a problem with the reliability of the connection section. Therefore, it is difficult to directly connect a flexible wiring body to an existing connector.
A possible method is to indirectly connect a flexible wiring body to a connector of a circuit board by connecting the flexible wiring body to one end of an existing wiring body such as a flexible flat cable (FFC) or a flexible printed circuit board (FPC) and connecting the other end of the FFC or the like to the connector of the circuit board. In this method, a flexible wiring body and an FFC or the like are bonded via a conductive adhesive or the like. As an example, FIG. 4 illustrates a cross-sectional view of a connection section of a flexible wiring body and an FFC. In FIG. 4, a state of the flexible wiring body before being extended is illustrated in broken lines. As FIG. 4 illustrates, a flexible wiring body 80 includes a base material 81 made of an elastomer and a wiring 82 containing an elastomer and a conductive material. An FFC 90 includes an insulating base material 91 and a wiring 92. A back end 83 of the flexible wiring body 80 and a front end 93 of the FFC 90 are bonded via a conductive adhesive layer 30.
As described above, in the flexible wiring body 80, the wiring 82 extends and contracts together with the base material 81. On the other hand, the FFC 90 does not extend and contract. Therefore, when the flexible wiring body 80 extends, at a forefront 930 of the FFC 90, a force acts in a direction that the FFC 90 peels off (see white arrow in FIG. 4). In the wiring 82 of the flexible wiring body 80, conductive particles are filled in the elastomer. A conducting path is formed through contacts between neighboring conductive particles. However, the conductive particles are not bonded or linked to each other. Therefore, as compared to the insulating base material 91 and the wiring 92 of the FFC 90, the wiring 82 has a small mechanical strength. Therefore, when the flexible wiring body 80 repeats extension and contraction, there is a risk that the wiring 82 may break in a section abutting the forefront 930 of the FFC 90 via the conductive adhesive layer 30.
When bonding the flexible wiring body 80 and the FFC 90 via the conductive adhesive layer 30, the two wiring bodies 80 and 90 are pressurized with a conductive adhesive in a paste form therebetween. However, the wiring 82 of the wiring body 80 has a small mechanical strength. Therefore, during the pressurizing, a corner of the forefront 930 of the FFC 90 abuts the flexible wiring body 80 via the conductive adhesive, thereby making the wiring 82 easy to break.